Quantum sensor replenishment

ABSTRACT

Atom-scale particles, e.g., neutral and charged atoms and molecules, are pre-cooled, e.g., using magneto-optical traps (MOTs), to below 100 μK to yield cold particles. The cold particles are transported to a sensor cell which cools the cold particles to below 1 μK using an optical trap; these particles are stored in a reservoir within an optical trap within the sensor cell so that they are readily available to replenish a sensor population of particles in quantum superposition. A baffle is disposed between the MOTs and the sensor cell to prevent near-resonant light leaking from the MOTs from entering the sensor cell (and exciting the ultra-cold particles in the reservoir). The transporting from the MOTs to the sensor cell is effected by moving optical fringes of optical lattices and guiding the cold particles attached to the fringes along a meandering path through the baffle and into the sensor cell.

BACKGROUND

Due to their high sensitivity, quantum sensors, e.g., gravimeters,gravity gradiometers, gyroscopes, and magnetometers, can provideimprovements over other state-of the art sensors. Typically, quantumsensors require populations of ultra-cold particles (e.g., neutral andcharged atoms and molecules) in quantum superposition. Obtaining areading from a quantum sensor typically requires driving sensorparticles out of quantum superposition so that subsequent readingsrequire newly generated quantum sensor particles. What is needed is aquantum sensor system that minimizes or eliminates the durations overwhich read-outs are delayed due to an insufficient sensor particlepopulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical-trapping continuousquantum sensor system.

FIG. 2 is an alternative view of the sensor system of FIG. 1 .

FIG. 3 is a view of some of the components of the sensor system of FIG.1 showing how optical traps are formed within the sensor cell.

FIG. 4 is a flow chart of an optical-trapping continuous quantum sensingprocess, implementable in the sensor system of FIG. 1 and in othersystems.

DETAILED DESCRIPTION

The present invention provides for optically trapping ultra-coldparticles to be used in a reservoir to replenish a sensor population ofparticles in quantum superposition continuously or promptly upon demand.While the use of a particle reservoir to feed the sensor populationeliminates or minimizes inter-readout latency due to particlepreparation, there remains the challenge of establishing and maintainingsuch a reservoir.

The present invention provides for a sensor cell as a site for thereservoir and, in most cases, the sensor population. The sensor cell canmaintain an ultra-high vacuum (UHV, correspond ng to pressures below10⁻¹⁰ Torr). An “atom chip” is an integrated circuit useful formanipulating quantum particles. Typically, an atom chip has anambient-facing face and a vacuum-facing face. Conductors on the vacuumfacing face can generate magnetic fields in the UHV interior of a sensorcell; the currents can be generated by electronics outside the UI-IV anddelivered to the conductors by conductive vias extending through theatom-chip. The magnetic fields generated by the atom chip can be used totransfer particles into and out from the reservoir, to confine particlesto the reservoir, and to cool particles to ultra-cold (<1 μK)temperatures.

High-energy particles that might otherwise interact with and heat thereservoir particles are excluded from the sensor cell. The particlesthat do enter the sensor cell are pre-cooled to yield particles below100 μK to yield “cold” particles. A pre-cooler can use near-resonant(capable of causing energy-level transitions in the particles) laserbeams to trap (with the help of magnets) and cool the particles. Theresulting cold particles can then be transported to the sensor cell.

The near-resonant light must be excluded from the sensor cell;otherwise, it could excite and heat the reservoir particles. In someembodiments, a baffle is used to trap any near-resonant light leakingfrom the pre-cooler so that it does not enter the sensor cell. Thetransporter can guide the cold particles along a meandering path throughthe baffle and into the sensor cell without allowing in thenear-resonant light.

The transport mechanism can use one or more optical lattices. An opticallattice is formed by interfering counter-propagating laser beams to formbright (constructive) and dark (destructive) fringes. Particles areattracted to and trapped by the bright fringes. By sweeping thefrequency of at least one of the counter-propagating laser beams, thefringes can be made to move, dragging the trapped particles along theoptical conveyor belt. The transport mechanism can use a series of twoor more straight-line-segment optical lattices to define a meanderingpath; alternatively, one or more multi-dimensional lattices can be usedto define a meandering path through the baffle into the sensor cell forultra-cooling, maintenance in the reservoir, and replenishment of thecontinuous quantum sensor.

As shown in FIG. 1 , a quantum sensor system 100 includes a vacuumenvironment 102, a laser system 104, and an imager 106 for capturingsensor data used to evaluate one or more parameters, e.g., accelerationor angular momentum. Vacuum environment 102 extends across a particlesource 112, a particle pre-cooler 114, a transporter 116 with a baffle,and a sensor cell 120. An atom chip 122, shown in FIG. 2 , serves as avacuum boundary wall of the sensor cell. In an alternative embodiment,all walls of the sensor cell are glass. In addition, the sensor cell caninclude an unloading (aka “supply”) region 124, a quantum particlereservoir 126, and a sensor population 128 of particles in quantumsuperposition. Particle pre-cooler 114 includes magnets 115, which maybe located outside vacuum environment 102.

Electronics 110, FIG. 1 , located outside sensor cell 120, interfaceswith the ambient side of atom chip 122. Voltages and currents generatedby electronics 110 can be applied to the vacuum-facing face of atom chip122 via conductive vias through atom chip 122. For example, currentsthrough vacuum-side conductors of atom chip 122 can establish andcontrol magnetic fields within sensor cell 120. These magnetic fieldscan be used to move, trap, and cool particles in sensor cell 120. In analternative embodiment, such magnetic fields can be produced by an atomchip external to the sensor cell. In the illustrated embodiments,optical fields are used to move, trap, and cool atoms within the sensorcell.

Laser system 104 provides near-resonant laser beams 130 of frequenciesdesigned to interact with the particles of interest. For example, theparticles can be neutral cesium atoms ¹³³Cs or rubidium atoms ⁸⁷Rb,other alkali metal atoms, atoms of other elements, Rydberg atoms, ions,or neutral or charged molecules. The laser frequencies qualifying as“near-resonant” vary according to the species of the particles beingpre-cooled. The near-resonant laser beams cooperate with magnets 115 toproduce magneto-optical traps (MOTs) used to confine, trap, and coolparticles in pre-cooler 114.

Laser system 104 also provides non-resonant laser beams 132.Counter-propagating pairs of non-resonant laser beams 132 interfere toproduce optical lattices with bright (constructive) and dark(destructive) interference fringes. Particles are attracted and thustrapped by the bright fringes. Sweeping a frequency of a constituentlaser beam can cause the fringes to move, dragging trapped particlesalong with the fringes. This phenomenon is leveraged in a transporter116 to transport particles from the pre-cooler 114 to sensor cell 120.

Probes 134, resonant or near-resonant laser beams, are generated bylaser system 104 to force particles in sensor population 128 out ofquantum superposition for evaluating one or more parameter values.Imager 108 captures the states for the particles driven out of quantumsuperposition.

As shown in FIG. 2 , pre-cooler 114 includes a 2D+ MOT 202 and a 3D MOT204. In other embodiments, a 2D MOT or a 2D-HP MOT can be used insteadof the 2D+ MOT. A 2D MOT (without the “+”) uses magnets to form aquadrupole that cooperates with laser beams to confine particlesradially, while allowing them to progress through and out of the 2D MOT.The particles can be laser cooled on their journey through the 2D MOT.The 2D+ MOT adds an optical molasses to slow particle progress and givemore time for laser cooling. In a variation, a 2D-HP MOT is used; inthis case, the optical molasses is implemented using hollow laser beams;the hollow is filled with push beams that control progress through theMOT independently of the optical molasses. This can allow optimizationof the time for laser cooling and the time to reach the 3D MOT.

The 3D MOT uses evaporative cooling to reach sub 100 μK temperatures toyield “cold” particles. Higher energy particles are allowed to escapethe trap, while lower energy particles remain. The result is that theaverage temperature decreases. In alternative embodiments, other 2D and3D traps can be used. For example, optical 2D and 3D traps can be usedto cool particles to yield cold particles.

Atom chip 122 and/or optical traps can cool cold particles to yieldultra-cold particles. However, the atom chip's ability to produce andmaintain particles at ultra-cold temperatures would be compromised bythe presence in sensor cell 120 of near-resonant light, e.g., that hasleaked from pre-cooler 114. Accordingly, the transporter 116 used totransfer particles from pre-cooler 114 to sensor cell 120 includes abaffle 206.

Baffle 206 is a multi-chamber light box including chambers 208 and 210.An entrance pin-hole 212 to chamber 208 provides a first barrier tonear-resonant light leaking from pre-cooler 114. Chamber walls arecoated to absorb near-resonant light so the walls of chamber 208 serveto attenuate any light entering via pinhole 212. An inter-chamberpinhole 214 serves as a barrier to near-resonant light escaping chamber208 to chamber 210. Absorbent walls of chamber 210 attenuatenear-resonant light entering chamber 210 from chamber 208. Finally, anexit pinhole 216 serves as a further barrier to near-resonant lightescaping baffle 206.

Transporter 116 uses a series 220 of optical lattices 221, 222, 223 tocarry cold particles along a meandering path through pinholes 212, 214,and 216 and thus through baffle 206 to sensor cell 120. Each opticallattice is formed using a respective pair of counter-propagating laserbeams, which interfere to produce bright and dark fringes; the particlesare attracted to and trapped by the bright fringes. Sweeping a phase orfrequency of one of the counter-propagating laser beams causes thefringes and, thus, the cold particles, to move. The optical lattices221, 222, and 223 are one-dimensional lattices that are constrained tomoving along straight-line segments. Changes in direction result fromhanding off particles from one optical lattice to another. Inalternative embodiments, multi-dimensional lattices are used fordirection changes to implement a meandering path through a baffle forcold particles. As shown in FIG. 2 , mirrors 230 are used to redirectlaser beams going into the page (z direction) to directions within theplane of the page.

After transportation by the optical lattices, the cold particles areoff-loaded at supply region 124 of the sensor cell. Atom chip 122 oroptical traps can confine the cold particles, e.g., using a harmonicmagnetic field or an optical field and then cool them using forcedradio-frequency (RF) evaporative cooling to yield ultra-cold particlesat a temperature below 1 μK and, in some embodiments, below 0.1 μK. Inthe illustrated embodiment, the particles are cooled to about 50 nk toform a Bose-Einstein condensate (BEC). The confining and cooling areachieved, at least in part, by optical fields or magnetic fields createdby currents through conductors 224 of atom chip 122.

Atom chip 122 or optical traps transfer the ultra-cold particles fromsupply region 124 to reservoir 126, which can also be confined by anoptical trap or a harmonic magnetic potential generated by atom chip122, in some embodiments, additional forced rf evaporative cooling isapplied to particles in the reservoir to maintain a desired ultra-coldtemperature.

In an alternative embodiment, transporter 116 uses an optical lattice todeliver cold particles directly to the reservoir. Traps can then coolthe cold particles to ultra-cold temperatures. However, off-loading thecold particles and cooling them at a supply region separate from thereservoir avoids any delay involved in the cooling when responding todemand from the sensor population for reservoir particles. In addition,the bright fringes of the optical lattice reaching the reservoir maycause undesirable interactions with the population already in thereservoir. Furthermore, off-loading the cold particles away from thereservoir allows control of both the inputs to and the outputs from thereservoir. The atom chip can delay an input to the reservoir if itconflicts with an output from the reservoir; in other words, the atomchip can delay an input (the timing of which is not critical) to givepriority to an output (the timing of which may be critical).

In some embodiments, some of the cooling is performed in the sensor cellbut outside the reservoir, while the remainder of the desired coolingtakes place in the reservoir. This reduces any delay due to cooling atthe reservoir, while leaving it to the reservoir to achieve the desiredtemperature. For example, the reservoir can be tasked with providing thefinal cooling required to form a BEC. Since multiple batches ofparticles can be represented in the reservoir at any given time, itcannot be assumed that the particles share the same state. Heating andre-condensing particles in the reservoir to re-form a unified BEC may beappropriate for applications requiring all particles in the reservoir tobe in the same state.

Evaporative cooling is a batch-mode process. Cold atoms leave the 3D MOTin hatches and ultra-cold atoms enter the reservoir in batches. Thereservoir acts as a buffer that accepts batch inputs and outputscontinuously or otherwise promptly upon demand to replenish the sensorpopulation.

The sensor population can include particles in a BEC or other ultra-coldstate. They may be confined by harmonic magnetic potentials generated bythe atom chip or by magnetic optical traps, or by optical traps (e.g., a3D optical lattice). The particles may be in quantum superposition,reacting to physical phenomenon. Laser probes 134 (FIG. 1 ) can interactwith the sensor particles to force them out of quantum superposition forimaging by imager 108. The particles lost due to the probing can beimmediately replenished by particles from reservoir 126.

All-optical traps can be formed at unloading zone 124, reservoir 128,and sensor population 128 using the laser system components shown inFIG. 3 . A ytterbium fiber laser 302 outputs to a collimator 304, whichfeeds an acousto-optical modulator (AOM) 306. AOM outputs 0^(th) orderand 1^(st) order beams 308 and 310, which are directed by mirrors 312through respective lenses 314 and 316 into sensor cell 120. Withinsensor cell 120, beams 308 and 310 intersect at a 19.5° angle to definean optical trap 320. Once beams 308 and 310 exit sensor cell 120, theyare disposed of using a beam dumper 320. A portion of 1^(st) order beam310 is picked off and detected by photo-detector 330, the output ofwhich is input to a proportional-integral-derivative (PID) controller332, which is used to control a radio-frequency (RF) driver 334 that, inturn, controls AOM 306. Alternatively, optical traps can be formed asdescribed in “All-optical ⁸⁷Rb Bose-Einstein condensate apparatus:Construction and operation”, a Ph.D. thesis by I. L. H. Humbert,submitted to The University of Queensland in 2012, School of Mathematicsand Physics.

A sensor replenishment process 400, flow charted in FIG. 4 , can beimplemented in sensor system 100 and in other systems. At 410, particles(e.g., Rydberg and other neutral atoms of Lithium 7, Cesium 133,Rubidium 87, or other alkali or non-alkali elements), ions, and chargedand neutral molecules) are released, e.g., by heating an ampule filledwith the species of interest.

At 420, particles are cooled to yield cold particles below 100 μK. Insome embodiments, this is a two-step process. First, particles arecooled in a 2D (which may be a 2D+ or 2D-HP) MOT to near a Dopplerlimit, and then cooled further by evaporative cooling in a 3D MOT toyield cold-particles.

At 430, a transporter transfers the cold particles to a sensor cell. Insome embodiments, the transport is via a meandering path through abaffle designed to prevent near-resonant light leaking from the 3D MOTfrom reaching the sensor cell. To this end, the transporter sweeps thefrequencies of laser beams used to form optical lattices so as to moveinterference fringes to which the cold particles are attracted.

Depending on the embodiment, the cold particles may be delivered to areservoir in the sensor cell directly or to a separate supply region inthe sensor cell for later transfer to the reservoir by optical ormagnetic fields of the sensor cell. In either case, at 440, the coldparticles are further cooled to below 1 μK to yield ultra-coldparticles. Depending on the embodiment, the temperatures may be below0.1 μK., i.e., below 100 nanoKelvin (nk). For example, the particles canbe cooled to about 50 nK to form a BEC. The cooling can be performed inthe reservoir or at the supply region or both. To cool the coldparticles, the traps can confine them and then use forced rf evaporationfor the actual cooling. The series of actions iron. 410-440 can berepeated, e.g., in a pipelined manner, for additional batches ofparticles.

At 450, a sensor particle population is replenished by transferringultra-cold particles from the reservoir to the sensor population. Insome embodiments, the transfer is completed under atom-chip control. Inother embodiments, at least some of the transfer can be effected usingoptical lattices and/or other optically generated devices such as bottlebeams.

At 460, the sensor population is “read out”, e.g., by using a probe beamto knock some or all sensor particles out of quantum superposition andimaging the resulting particle distribution to evaluate a parameter,e.g., angular momentum or acceleration. Since this read out depletes it,the sensor population can be replenished as process 400 returns to 450to begin the next replenishment iteration.

As used herein, the term “particle” is equivalent to “molecular entity”as defined in the International Union of Pure and Applied Chemistry(IUPAC) Goldbook to mean: “Any constitutionally or isotopically distinctatom, molecule, ion, ion pair, radical, radical ion, complex, conformer,etc., identifiable as a separately distinguishable entity.”

Herein, “cold” refers to temperatures below 1 milliKelvin (1 mK), and“ultra-cold” characterizes particle temperatures below 1 μK. Dependingon the embodiment, the ultra-cold particles can further be below 0.1 μK,otherwise expressed as 100 nK. For example, in an exemplary BEC, thetemperature can be about 50 nK.

Herein, “cooling” can include laser cooling, polarization gradientcooling, Doppler cooling (e.g., involving optical molasses), evaporativecooling (including forced radio frequency evaporative cooling), andcombinations thereof. Much of the cooling occurs with particles in“traps”, including 2D, 2D+, 2D-HP, and 3D magneto-optical traps, opticaltraps including 3D lattices, and harmonic magnetic potential traps,e.g., established on an atom chip.

Herein, a “baffle” is a structure designed to trap light, e.g., toprevent it from reaching some destination, such as a sensor cell from a3D MOT cell. For example, in the illustrated embodiment, a baffledefines a narrow meandering (non-straight) path through a baffle suchthat very little light entering the baffle can follow that meanderingpath and escape the baffle. In some embodiments, a baffle may absorblight that hits baffle walls; in other cases, light may be directed awayfrom the destination to be protected from the light. In the illustratedembodiment, it is the near-resonant light escaping a 3D MOT cell that isto be prevented from reaching a sensor cell.

Herein, “meandering” means “not straight”. A meandering path can includea series of two or more straight line segments arranged to cause thecarried content to change directions en route to their destination; suchpaths can be implemented using a series of one-dimensional opticallattices. Also, meandering encompasses paths with curves; such curvedpaths can be implemented using multi-dimensional optical lattices. Anoptical lattice can be established by interfering counter-propagatinglaser beams, which produces constructive (bright) and destructive (dark)fringes. Particles may be attracted and thus trapped by the brightfringes. Sweeping phase and/or frequency of at least one of thecounter-propagating beams can cause the fringes to move, carrying thetrapped particles along. Thus, the optical lattices function as anoptical conveyor belt for the particles.

Herein, “quantum physics sensor” refers to devices that use particles insuperposition to respond to physical stimuli (such as heat, light,sound, pressure, magnetism, or a particular motion) and transmit aresulting impulse (as for measurement or operating a control). “Quantumcomputational sensor” refers to devices that use particles insuperposition to represent an intermediate or final quantumcomputational result. Herein, unless it is clear from context that theterm “sensor” encompasses a quantum computational sensor, “sensor” means“quantum physics sensor”. Herein, “replenish” means “to add to anexisting population so as to return it to a greater previous quantity.”

Herein, art labelled “prior art, if any, is admitted prior art; art notlabelled “prior art” is not admitted prior art. The illustratedembodiments, variations thereupon, and modifications thereto are withinthe invention's scope, which is defined by the following claims.

What is claimed is:
 1. An optical-trapping quantum sensor replenishmentcomprising: cooling, in an optical trap of a sensor cell, pre-cooledparticles having a temperature below 100 microKelvin (μk) to yieldultra-cold particles below 1 μk, wherein the cooling provides theultra-cold particles in a reservoir in batches; maintaining theultra-cold particles in the reservoir within the sensor cell; andtransferring the ultra-cold particles from the reservoir to replenish asensor population of particles, wherein the transferring includescontinuously transferring the ultra-cold particles from the reservoir tothe sensor population during a sensor use.
 2. An optical-trappingquantum sensor replenishment process comprising: cooling, in an opticaltrap of a sensor cell, pre-cooled particles having a temperature below100 microKelvin (μk) to yield ultra-cold particles below 1 μK, whereinthe cooling provides the ultra-cold particles in a reservoir in batches;and maintaining the ultra-cold particles in the reservoir within thesensor cell; and transferring the ultra-cold particles from thereservoir to replenish a sensor population of particles, wherein thetransferring includes transferring the ultra-cold particles from thereservoir to the sensor population in response to a sensor demand.
 3. Asensor system comprising: a sensor cell; a laser system for establishingat least one optical trap within the sensor cell to cool pre-cooledparticles from below 100 microKelvin (μk) to below 1 μK to yieldultra-cold particles and to maintain the ultra-cold particles in areservoir; and a sensor replenisher for transferring the ultra-coldparticles from the reservoir to a sensor population of particles inquantum superposition, wherein the sensor replenisher transfers theultra-cold particles from the reservoir to the sensor populationcontinuously during a sensor use or in response to a sensor demand.